The present invention generally relates to an optical information recording medium, and a recording and reproducing method for recording and reproducing information with high speed and high density.
When a laser beam is focused by a lens system, the diameter of optical spots can be made smaller, on the order of the wavelength of the light. Therefore, light spots having a high energy density per unit area can be made even from a low level light source. Minute regions of a material can be changed by such light spots, and the changes in the minute regions can be read out. An optical information recording medium uses such changes in recording and reproducing operations. The optical information recording medium is hereinafter referred to as "an optical recording medium" or simply "a medium".
A recording film layer is provided, in the basic construction of the optical recording medium, on the substrate whose surfaces are flat. The recording film layer changes in some condition by the application of laser light spots. The signal recording and reproducing operations on the medium are effected by the method described hereinafter. The recording medium is moved by a rotating means and a translating means such as a motor so as to apply focused laser beams onto the recording film face of the medium. The recording film absorbs the laser beams so as to raise its temperature. When the output of the laser beams is at least a threshold value, the condition of the recording film is varied so as to record the information. The threshold value is a quantity depending upon the thermal characteristics of the substrate, in addition to the characteristics of the recording film itself, and the relative speed of the medium with respect to the optical spots. Laser beam spots of an output sufficiently lower than the threshold value are applied to the recording portion of the medium and the differences of some optical characteristics in one of the transmission light intensity, the reflection light intensity, the polarization directions or the like between the recording portion and the non-recording portion are detected for reproducing the recorded information.
A metallic film with Bi or Te as principal components, or a compound film including Te are known recording films. They are shape change types of recording mediums using the steps of melting or evaporating the films with a laser beam, and forming small holes. The signal reproduction from the recorded portion is effected by the detection of the difference between the amount of light reflected or the amount of light transmitted between a small hole portion and the peripheral portion thereof.
A state change type of recording film has optical changes without being accompanied by the shape changes. The state change type recording film changes its state condition by the application of a laser beam, and changes its complex refractive index during the time. Generally the refractive index n and the extinction coefficient k of the complex refractive index change in the same direction. Most materials considered as optical recording mediums increase the complex refractive index when the state condition changes to a crystalline state from an amorphous state. A weak light is applied to the signal pattern formed as the difference of the phase condition and the amount light transmitted or reflected from the medium is measured so as to effect the signal reproducing operation from the recording film.
Light is described by amplitude and optical phase. The information from the recording medium is effected by the detection of changes in the amount of light transmitted or reflected to a photodetector of the reproduction optical system. There is a case (amplitude change record) where a transmission beam amplitude or a reflection light amplitude in the minute region of the optical film itself changes, and a case (optical phase change record) where the phase of the transmitted light or the reflected light changes. Reproduction signals are obtained by the change in the complex refractive index by the state change being provided as the composition of the changes of both the amplitude change and the state change.
The phase change optical disk records signals by the formation of the difference (recording mark) of the local state condition on the recording film, by the application of a laser beam modulated in intensity on the rotating recording medium, and reproduces the signals with the detection, as a reflection difference, of the difference caused between the conditions. The size of the recording mark to be obtained becomes a size of the focusing optical spot, namely, on the order of a wavelength. Assume that a laser beam of approximately 780 nm in wavelength is focused using a lens system of approximately 0. 5 in N.A. (numerical aperture), and the full width half maximum in intensity is focused to spot of approximately 0.9 .mu.m. The intensity of the optical spot is generally of a Gausian distribution or a distribution of a shape which is closer to it. When a recording operation is effected using the optical spot, the state condition become a recording condition with the range of approximately 5 through 1 .mu.m being changed.
FIGS. 1(a)-1(c) show the relationship between the recording mark and the optical spot for obtaining the maximum signal change. In FIG. 1(a) the state change recording film is adapted to show the amplitude change record, and in FIG. 1(b) it is adapted to show the optical phase change record. In the construction showing the amplitude change records FIG. 1(a), the amplitude change recording mark 3 formed on the recording film 2 on the substrate 1 changes mainly in reflection index. In the scanning operation on the mark 3 by the optical spot 4 formed by a weak light beam I.sub.0 for reproduction, the amount of light change on the photodetector to be obtained by the reproduction optical system, that is, the conditions for making the signal amplitude maximum, namely, the conditions for making maximum the difference between the reflection light l.sub.2 of the recording mark portion and the reflection light l.sub.1 of the non-recording condition are to make the size of the recording mark 3 changed state condition is equal to or greater than the reproduction spot 4 size.
As in FIG. 1(b), even in the case of the optical phase change recording operation, the change of the recording film 2 itself is the same as the amplitude change, and the recording mark 6 is formed by the similar beam application. In the recording mark showing the ideal optical phase change record, the reflected light I.sub.4 of the same intensity as that of the reflected light I.sub.3 of the non-recording condition is reflected with respect to the incident light quantity I.sub.0 and changes by .phi. in the optical phase of the light. The recording condition by the optical phase change forms concave or convex pits 7 on a plane portion as shown in FIG. 1(c) with respect to the optical spot, and functions as when the optical phase has changed by the concave or convex stage difference. The conditions showing the maximum signal amplitude by the optical phase change records become conditions where the diffraction effect of the light by the optical phase difference by the optical phase change record is conditions where the diffraction effect of the light by the optical phase difference when the reproduction spot 4 has scanned the recording mark becomes maximum. When the optical intensity of the region incident to the recording mark 6 becomes equal to the amount of light incident on the peripheral portions of the optical spots 4, the effect of the interference becomes largest and the amount of light into the photodetector becomes minimum. The amount of reflected light becomes minimum under the cancelling conditions with interference between the reflected light I.sub.4 from the recording mark and the reflected light I.sub.3 from the non-recording portion, so that the maximum signal amplitude of the optical phase change record is obtained.
When the two recording modes are compared with the recording marks showing the maximum amplitude, it has been found that the recording and reproducing operation of high density in the optical phase change recording can be effected, because the optical phase change recording mark 6 can be recorded in a smaller shape than the amplitude change recording mark 3. It is possible to provide a recording medium which is interchangeable with an optical disk for reproduction only use where concave and convex pits like a compact disk or the like are used for recording information if the optical phase change recording can be realized.
As the state change medium showing the optical phase change recording is a recording method using a heat mode using the heat of the light, the application of the light to the recording film is accompanied by a thermal diffusion phenomenon. A portion which has absorbed the optical energies rises in temperature and also, at the same time, the generated heat is diffused to a portion where the peripheral temperature is lower. The recording mark formed in the application portion is distributed in the intensity of the light to be applied, with a problem in that the size thereof changes in accordance with the amount of energy (application power) to be made. In the case of the conventional reflected light change, the mark shape effects a maximum signal amplitude with the mark shape being equal to the spot diameter (a size which becomes 1/e.sup.2 in the intensity of light). A pitch of the mark to be recorded in the track direction is a value equal to a spot diameter or a lower value from a point of view of raising the recording density as much as possible. It is necessary to make the recording mark constant in range for the stability of the signal reproduction from the recording portion. It is necessary to set the energy of the light applied at the recording time, namely, the recording power within a constant range. When the change in the mark shape is large with respect to the recording power, the power range becomes small. It becomes very difficult to design a recording apparatus including the variation and dispersion and so on of the recording laser light. The optical phase change record is suitable for the high density recording, because the mark shape is relatively small as compared with that of the amplitude change recording method. In order to make small recording marks, it is more difficult to retain within a constant tolerance range the tolerant width, namely, the recording power with respect to the recording variation.